An apparatus adapted to measure residual stress of a measuring object using X-rays is described in Patent Literature 1. The apparatus comprises an X-ray emitter adapted to emit X-rays to a measuring object, an imaging plate adapted to receive diffracted light from the measuring object, a rotating mechanism adapted to rotate the imaging plate, a laser device adapted to take a reading from the imaging plate, and a controller adapted to control these components.
The controller stores geometries of diffraction rings corresponding to residual stresses as references in advance. Then, the apparatus receives diffracted light from the measuring object using the imaging plate, acquires a diffraction ring by reading received light intensity with the laser device while rotating the imaging plate with the rotating mechanism, and compares the geometry of the acquired diffraction ring with the geometries of reference diffraction rings. Then, the apparatus calculates the residual stress corresponding to the diffraction ring of the closest geometry as the residual stress of the measuring object. When a measured diffraction ring is discontinuous, the apparatus calculates the residual stress by cosα method based on the geometry of the diffraction ring.